Thermionic tube control system



Aug. 12, 1952 H. B. w. HOLT 2,607,005

THERMIONIC TUBE CONTROL SYSTEM Filed May 7, 1948 s Sheets-Sheet 1 IN VEN TOR Aug. 12, 1952 5, w 353 2,607,005

THERMIONIC TUBE CONTROL SYSTEM Filed May 7, 1948 3 Sheets-Sheet 2flerberf Bruce W/Mo/zi //0/7.

A? 4- w W4? A ORZYS Aug. 12, 1952 H. B. w. HOLT 2,607,005

THERMIONIC TUBE CONTROL SYSTEM Filed May 7, 1948 s Sheets-Sheet s //VVE/VTOR TTORNEYS Patented Aug. 12, 1952 THERNHONIC TUBE CONTROL SYSTEMHerbert Bruce William Holt, Burpham, Guildford, England, assignor to P.A. M. Limited, Westminster, England, a British company g Application May7,1948, Serial No. 25,583

, In Great Britain October 30, 1946 6 Claims.

The invention relates to improvements in means for controllingtemperatures of the type in which thermionic valve amplifying means areemployed for controlling temperatures to a fine degree of accuracy. Thechief object of the present invention is to provide temperaturecontrolling means with thermosensitive switches whose dimensions can beconsiderably smaller than existing types and in which the switch con- Itacts need not be sealed in evacuated envelopes 'to' ensure stableperformance, and to provide means whereby such small sensitive switchescan be connected to the valve amplifying means by thin electricalconductors of any length, without producing harmful effects in theamplifier by reason of voltages which may be induced in such long leadsfrom external magnetic fields.

With the amplifying means about to be described, the movements of thecontacts actuated by bimetal strips or by'rod and'tube type bimetalelements which are required to operate the valve amplifier are extremelysmall, being of the order for .000002". For this reason, bimetal stripsno rent in the thermostat circuit can be kept low but I longer than.25", or tube and rod elements no longer than 1" and 1%" diet. can beused to obtain temperature control to within 0.l C.

Since such small movements are sufficient to actuate thefampliflerspecial range control devices are required so that small enoughmovements of the adjustable contacts in the thermosensitive elements canbe easily made by manual operation of a dial or knob.

In order that such small movements of contac'ts' in the thermo-sensitiveswitches can be successfully used to actuate the amplifier con-"sistently, it is necessary that no arcing should take place at thosecontacts and that dust pari ticles in the vicinity of the contactsshould not f become electrically charged and adhere to the contactfaces. It is also important that slight variations of the contactresistance should not "aifect the behaviour of the amplifier.

To prevent arcing it is necessary to arrange that the current ,broken bythe contacts in the ther'mosensitive elements shall be very small, say,0.05 milliamp. or less. To prevent the charging of dust particles it isnecessary that the voltage across the contacts be kept low andalternating, 0.5 volt R. M. S. is found to be a satisfactory :.-value.Since it isdeslred to connect the sensi- -,tive element to theampliflerby leads that may be very long, the frequency of alternation of thevoltage must be kept ata value low enough to preventthe self capacityofthe leads from aflecting the -.operation of theamplifier, and the,frethe voltage in that circuit need be only a fraction of 2. volt.

According to the present invention the means for controllingtemperatures of the type having a sensitive element and valve amplifyingmeans controlling a heater through the medium of a relay includes apotentiometer consisting of a pair of resistances in parallel with theheater of the valve and is characterised in that the thermosensitiveswitch is so connected in the circuit that when its temperature israised one of the resistances of the potentiometer'is short circuitedand the grid of the input valveof the said amplifying means is connectedto earth, the signal applied to the grid of the valve when the contactsof the thermo-sensitive switch are open being amplified and rectifiedand the negative D. C. potential thus produced cutting off the cathodecurrent to a second valve the cathode or anode circuit of this latterhaving the relay and is further characterised in that an amplifyingvalve is provided between the valve controlling the heating circuit andthe sensitive element.

The invention will now be described with reference to the accompanyingdrawings in which:

Fig. 4 is a similar view of a modified construetion and Fig. 5 is alongitudinal section of a thermosensitive element suitable forcontrolling for instance room temperatures.

As shown in Figure 1 one form of the apparatus for remotely controllingtemperatures comprises a sensitive element 4 conveniently consisting ofa tubular member 5 and a rod member 6 secured at one end and adapted toengage a contact member I and leads 8 connecting the tubular member sand the contact member I to valve amplifying means adapted to actuate arelay I controlling contacts 2 and 3 of a switch of a heating device ofany suitable form and for any desired purpose. These contacts 2 and 3control the current supply from 50 cycle A. C. mains a. and band theheater connected to the leads x and v. The circuit arrangement includesthree valves VI, V2 and V3. size and may take several forms to behereinafter described in more detail." In one form the tubular member 5is constructed of me'tal'having a low co-efficient of expansion andsecured at" one end of thi member 5 is the rod member 5 having a highco-eflicient of expansion. The free outer end of the rod 6 is adapted onexpansion to contact the contact member 'I mounted in an extension ofthe tubular member. Conveniently this contact member I is connected toan inner conductor of a flexible single core screened cable and theouter coil of the cable isconnected to the rod 6 through the medium ofits tubular casing Obviously the remote connection to the amplifyingmeans may be efiected by a two core screened or unscreened cable or asingle core unscreened cable using earth connection as the return. Theend of the rod 6 is formed with a flat face ground and polished and theend of the contact member I is preferably ground spherical and polishedand the arrangement is such that an extremely small increase or decreasein the length of the rod member 6 will make and break a current of a fewmicro-amperes at a low voltage such as 0.5 volt without sparking ordamage to the contact faces.

The operation of the amplifier shown in this diagram is as follows:

The cathode heaters of the valves are con nected in series with theresistance R9 and to the mains a and b, one end of the heater of thevalve VI being joined to the earthed side I) of the main. In parallelwith the heater or" the valve VI is connected a potentiometerarrangement consisting of resistances RI and R2. The sensitive element 4is so connected that when its temperature is raised to the predeterminedvalue, R2 is short circuited and the grid of the valve VI is connectedto earth. Before the sensitive element is heated it will be seen that a50 cycle A. C. signal is applied to the grid of the valve VI. Thissignal is amplified and the amplified signal at the anode of the valveVI is supplied to the diode anode in the envelope of the valve VI by thecoupling arrangement Rd and C2. The resistance R3 and condenser CI arethe bias resistance and decoupling condenser for the valve VI. Theamplified signal applied to the diode anode is rectified by the diodethe resistance R5 being its load resistance, and the negative D. C.potential thus produced between the diode anode and. the cathode isapplied through the filter network R6, R1, and C3 to the grid of thevalve V2. This negative potential is sufiiciently large to cut offcompletely the cathode current of the valve V2. V2 is a valve of such atype that its cathode current with normal bias is of the order of 40-50milliamperes which current is sufiicient to operate the relay I includedin the cathode circuit of the valve V2.

The sensitive element i is of small the heating device, the temperatureof which is desired to be controlled, it will be seen that until theexpanding rod 6 in the sensitive element 4 has short circuited the gridand cathode of the valve VI the relay contacts remain closed, and heatis generated in the heating device. When the temperature reaches thevalue at which control is desired, the expanding rod 6 in the sensitiveelement 4 makes contact with the member I and short circuits the gridand cathode of the valve VI and no signal is applied to the diode. The.grid of the valve V2 is thus deprived of the negative biaswhich waspreventing flow of cathode current and so the cathode current of thevalve V2 rises until the coil of the relay I is sufficiently energisedto open the contacts 2 and 3 This relay I is 50 arranged that with nocurrent X in its winding the contacts Z'and 3 are closed and since thesecontacts are connected between the mains a and b and the leads :2 and yto of the heater circuit. Since the relay coil is in the cathode circuitof the valve V2 its resistance is made to provide the normal operatingbias for the valve and prevents the cathode current from becoming largeenough to damage the valve. As the heater cools downthe sensitiveelementcontacts 6 and I open and inturn the relay contacts 2 and 3 close andfurther heat is generated by the heating device.. The cycle repeatsitself'and so the temperature of the heating device is controlled. TheresistanceR8 is notrequired for some types of valves. The valve V3 ispurely a rectifier providing D; C. anode current for the other valves,C4 being the reservoir condenser for the valve V3. Because of the low'inputimpedance of the valve VI the connection 8 between the sensitiveelement! and the amplifier can be of any length andunscreened withoutany undesirable effects being roduced by the proximity of A. C. mainscables to the sensitive element lead.

In the form shown 'theidiagram, Figure 1, universal valves are usedforthe sake of cheapness, but the amplifier could be constructed tooperate using a'mains transformer and A. C. mains valves, such analternative amplifier of lower consumption and arranged to isolate thesensitive element'from the mains ab will now be described with referencetoFig. 2 of the accompanying drawings in which the same referencenumerals are employed 'forsimilar' parts:

In this diagram. the thermo-sensitive element 1 is depicted as a pair ofelements'fi and 6 of himetal type to provide a'make andbreak contact 1instead of members having different co-efficients of expansion. I

This circuit also consists of the valves VI, and

V2 which together with all the resistors and condensers except C4constitute the operative parts of the amplifier; The main transformer'T,a

, metal rectifier M and a capacitor C4, together is smoothed by thefilter circuit R6, R1 and C3.

To considerthe circuit operation let us ignore to begin with theresistances RI' and -R2. If a low frequency alternating voltage or L. F.signal from any source is applied between. thegrid of 'VI and earth,amplified voltagelappea'rs .at. the

anode of VI is transferred via C2 to thediode section anode where it (isrectified and the smoothed D. C.- negative-pot'ential thus-produced i isapplied'to'the gridof; 'm the anode-circuit of V2 is connected theoperatingcoil-of-the relay, the contacts of which are used to switch onor off the heater circuit. It will'be clear that if a signal ofsuflioiently large amplitudefrom any source is applied'between the gridof the valve VI and earth, the valve V2 will be biased negatively at itsgrid and the anode current will be cut on, thus de-energising the relaycoil. If, however, the valve VI is deprived of the signal by some means,the grid of the valve V2 will be unbiased and anode current will flow,thus energising the relay coil.

Thus the thermo-sensitive switch 4 arranged to deprive the valve VI ofits signal when a predetermined temperature is reached will in turnenergise the relay coil I and open the relay contacts 2 and 3 cuttingoff the energy supply to the object or chamber being controlled.-

Let us now consider the method by which a suitable signal can begenerated and the contacts of a suitable thermo-sensitive switch can bemade to supply or deprive the grid of the valve VI of the signal. It hasbeen previously shown that a suitable signal should be of amplitudearound 0.5 volt R. M. S. and of a convenient low frequency. Such asignal is derived from the arrangement of the resistances RI These tworesistances are connected in series across the heater winding of themains transformer which is wound to supply 6 volts to the cathodeheaters of the valves. One end of this winding is connected to the earthline and the junction of the resistances RI and R2 is connected to thegrid of VI. The other end of R2 is also connected to the earth line sothat if-the valves of RI and R2 are chosen so that ens-. R2 0.5

a to cycle voltage of .5 volt R. M. s. will be ape plied between grid ofthe valve VI and earth.

If leads are taken from the grid of the valve VI and earth and connectedto the contactsvof a thermosensitive switch in such a way that closingof these contacts will short circuit R2, the valve VI can be deprived ofthe signal. to limit the current that will flow in the contact circuit,RI should be as large as possible and since .05 milliamp. is a suitablevalue for this current the value of RI is found from the relation fromwhich RI is calculated to be 120,000 ohms. From the previous relation R2is found to be 10,909 ohms. By increasing the values of RI and R2 in thecorrection proportion the signal voltage can be maintained at .5 and thecurrent reduced to a lower value. It is convenient, however, to keep R2as small as possible in order that it will swamp any voltages which maybe induced in the leads connecting the thermosensitive switch to thegrid of VI, if those leads are of considerable length and lie in closeproximity to and parallel with A. C. supply mains or other source ofalternating magnetic fields.

It will be appreciated that it is not necessary to deprive the grid ofvalve VI of all the signal volts in order to reduce the bias on the gridof V2 to a sufiiciently low value to allow enough anode currentto passto operate the relay. For this reason the contact resistance of thethermosensitive switch contacts and the resistances of the leadsconnecting the switch to the valve VI need not be zero ohms.

In order and R2. ii

ed on the boss 20.

It is possible therefore for the length of leads to beconsiderable andthe contact resistance can varywithin a few ohms without upsetting theoperation or accuracy ofthe'control.

It will be obvious that since the operation of the device consists inmaking and breaking a pair of small contacts by the use of heat thesensitive element may take various forms for different purposes and ifdesired they may be made adjustable. v

In the apparatus above described the sensitive elements are made presetto control any desired temperaturebut if desired a range controldevice'may be fitted'to enable the user to'select any temperature withina reasonably long range. One form of range control device is shown inFig. 3. As previously described in the first arrangement thethermo-sensitive switch '4 comprises the tubular member 5 and the rodmember 6 secured at one end and adapted to engage the contact member I.The range control means are conveniently fitted between this sensitiveelement 4 and the leads 8 leading to the amplifying means and'comprisesa metallic tube I0 in the end'of which are fitted insulating bushings IIand I2; The bushing II at one end is secured and the, bushing I2 at theother end is free to slide. The bushing I2 has a short metal rod I3secured through its centre and this rod I3 provicles the contact I. Theother bushing II has also secured therein a rod I4 and the inner oropposed ends of these rods I3 and I4 are connected together by a pieceof fiat steel spring I5. Conveniently the fixed bushing l I is formedwith a flange I6 on the outer end and held agains't'the tube ID by aunion II screwed on the tube I0. This union connects a cable screen I8to the tube I0 and is soldered or otherwise secured to provide one ofthe leads. An inner conductor I9 of the cable I8 provides the other leadand is connected to the outer rod I4 carrying the spring I5 so that theother rod I3 provides thecon'tact'l previously mentioned. Thus thethermo' sensitive switch is electrically connected to complete a circuitwith the amplifying meanswhen the end of the rod member 6 of thesensitive element contacts with the rod I3. Engaging in a fine pitchthread cut in a boss 20 extending at right angles in the wall of thetube I0 is a screw 2| fitted with an insulating tip 22 which presses onthe centre of the fiat spring I5. On the outer end of the screw 2I ismounted a manipulating disc 23 conveniently in the form of a dialgraduated in degrees of temperature to be read against a fixed pointeror arm 24 mount- The boss 20 is fitted with a gasket 25 suitably packedso as to render the range control device watertight. The contact face ofthe rod 6 is preferably ground fiat polished and plated and the contactface '1 is ground spherical polished and plated as in the previousconstruction so as to provide for minute actuation. The enlarged end ofthe tubular member 5 is threaded at 26 for screwing for instance in acylinder of a thermo-plastic moulding machine. In operation it will beseen that by turning the screw 21 to indicate the temperature desiredand shown on the indicator the fiat spring I5 will be bowed and thesliding insulating bushing I2 and rod I3 forming the contact I will bedrawnup the tube I0. Since the spring I5 is deflected through a smallangle only, a considerable movement of the screw 2| produces a smallmovement of the contact I and thus the make and break gap between thecontact I and the high. expansiohiant "ma. 3' can adjusted minutely;Obviously] any other suitable means such as cam' device's Tina"Ibeemployed, for bowingthespring'lfi." 1

In a modified constructionas shown in Fig. 4 the rod 6" is of lowexpansion alloy and is secured .atit's outer.endirilasleeveor tubularmember of high expansionallo'y. This member 5 is secured at'right anglesin the end of a tube .21 carrying the range control means; Thisconstruction is particularly suitable for use in a thermoplasticmoulding machine and the tube 2'! is threaded at 28 for this purpose sothat the sensitive element will'lie in alignment with and in anape'rtureformed, in the cylinder wall of the machine. Within the tube 21 isfulcrumed at 29 a lever 30the lower end of which is adapted to contactwith the end of the rod 6. This lever 30 is pressedb'y a coiled spring31 to engage a spring strip 32. The spring 3| is adjustable andconveniently mounted on an adjusting screw 33 threadedinto the side ofthe tube 21. The spring strip 32 is fitted with a contact 34 toco-operate with another contact 35 of a further spring strip 36. Thespring strips 32 and 3e are mounted in an'ins'ulatingplate 31 fitted inthe tube 21 and the ends of these strips provide tags 38 and 39,respectively for the connection of the leads not shown to the amplifyingmeans. The tube 27 is fitted with an insulating sleeve 40 and with aclosure cap ill. The range control means comprises. a control spindle'42 threaded into a boss 43 on the side of the tube 21. This spindle 42is fitted with an insulating button 44 to contact with the strip 36whereby the distance between the contacts 34 and 35 can'be adjusted. Theouter end of this spindle 42. is fitted with a suitablecalibrated'dialsuch as "that described in the previous construction and the operationto vary the range of temperature at which the device willo'perate' willbe readily understood.

Figure'S shows a fu'rther form of a thermosensitive element for airtemperature control. In this' construction a pair of bimetal strips 45are mounted in an insulating block 45 fitted in the end of aj'protectingsleeve 41 preferably formed of copper. These strips 45'are fitted withco-operatin'g'contacts.48 and with terminal tags 49 forjco nn'ectionwith the leads not shown to the amplifying means. The sleeve 41 isprovided with a terminalcover.51! fitted with an insulating sleeveffilsIt'wi ll be understood that when the strips le'are' actuated by thesurrounding temperature to close the contacts 48 the amplifying meanswill be brought into operation to actuate the relay as previouslydescribed. If desired means such as an adjusting screw may be providedto 'varythe distance between the contacts for temperature range control.

What I claim is:

l. Controlling means "operable in response to temperature changescomprising an electronic circuit includihgja thermo-sensitive switch, apluralityoff'valvesjand' a load device, th first valve being of the:diodetriode type and having its grid connected to one element of theswitch, its cath'odeconnectcd'to ground'and its plate connected to boththe power source through a load resistor, and t'o'its diode plate bymeans of a condenser, the second valve ,being of the triode typehavingits grid connected to the diode of the'fi'rst valve, saidconnection including a filter networklcomprisirig a 'pairof resistorsin. series aeoaoos a condensen'th cathode of'the second valve beingconnected to ground and th plate to the power source with the loaddeviceinterposedtherein, a pairor" resistors in series and connected inparallel with the: cathode-heaters of the valves, the midpoint of thelast named resistors being connected to thegrid of the first valve,whereby-when the switch is open, the signalapplied to the first valve isamplified and rectified and applied to the grid of the secondvalve'biasingthe second valve to limit the current therethrough to avalue below that required to operate the load device, and when theswitch-is closedby thermal action, the grid of the first valveis-connected toground, thus removing the signal from the grid of thesecond valve allowing the necessary current therethrough to operate theload device, V V

2. Controlling means as claimed inclaim 1 wherein the thermo-sensitiveswitch is I, in the form of a tubular member having a low coefficient ofexpansion, 2. rod-member having a high coefficient of expansion carriedwithin said tubular member, and a contact member connected with thefirst valve, said rod member being held in spaced relationship tosaidcontact member below the adjusted temperature of said switch. v

3. Controlling means as claimed in claim 1 wherein the thermo-sensitiveswitch comprises a tubular member carrying a rod, each having differentcoefficients of expansion, a housing upon which said tubular member androd are mounted, a fiat steel spring in said housing having one endconnected to a contact member slidably mounted in an insulating block inthe housing and its other end connected to a line leading to the firstvalve, the other line being connected to the tubular member, saidslidable contact being disposed adjacent but out of contact with saidrod except at normal temperature, and means mounted on the housing forcontacting the springintermediate its length to 'bow 'it and therebyadjust its length to vary'the normal temperature position of saidslidablezcontaet relative to said rod. V 4.' Contro11ing' meansf'asclaimed in claim 3 wherein the means for adjusting the length of thespring comprises a manually. operablafinely threaded member carried bythe housing at right angles to said. spring and a calibrated dialmounted on said threaded member by means of which it may berotated.

5. Controlling means as claimed in claim 1 wherein the thermo-sensitiveswitch comprises a tubular member having a high coefiicient ofexpansion, a, rod member of-low coefiicient of ex-. pansion carriedbysaid tubular member, a tubular housing. carrying. said tubular memberand being atright angle s thereto, a spring pressed lever iulcrumed-in;the t ubul-ar housin g and adapted to be engaged at oneend by aidrod,.'and"5,' air of spring strips mounted within said tubular housinghaving cooperating. contacts and being connected-to saidfirstvalve, oneof said strips adapted to be engagedby the other end'ofsaid lever, and.adjusting'means carried bythe tubular housing and engaeingthe other ofsaid strips whereby the distance between the contacts can be regulated.

-6. Controlling means according to claim 5 v wherein an adjusting'screwis carried by said housing and carries a coiled spring for engaging saidlever whereby pressure on the lever may be adjusted. Y Y

V iiERBERT BRUCE WILLIAM. HOLT.

(References on following page)" j 9 REFERENCES CITED The followingreferences are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Geissinger Nov. 9, 1909 PoinanApr. 25, 1911 Gerdien Dec. 19, 1933 Palmer Oct. 13, 1936 Hargreaves Mar.14, 1939 Ewertz May 28, 1940 Harrison Dec. 3, 1940 Number Number NameDate Albin Nov. 13, 1945 Schick Dec. 24, 1946 Sepavich Nov. 25, 1947Darby Dec. 9, 1947 Sepavich Feb. 17, 1948 Parcaro June 15, 1948 Ray Aug.10, 1948 FOREIGN PATENTS Country Date France Jan. 12, 1939

